1. Field of the Invention
The present invention relates to a spring steel excellent in fatigue characteristics. This spring steel yields springs such as engine valve springs, clutch springs, and brake springs which need outstanding fatigue characteristics.
2. Description of the Related Art
There has been an increasing demand for automobiles with lighter weight and higher output, and this demand necessitates development of engine valve springs and suspension springs that endure high stress. These springs are required to have good fatigue resistance and sag resistance so that they can support a large load stress. Valve springs are particularly required to have good fatigue strength, and this requirement is hardly met even with SWOSC-V (JIS G-3566) which is said to be best in fatigue strength among conventional steels.
The steel wire for springs which needs high fatigue strength is required to contain a minimum amount of hard nonmetallic inclusions therein. This requirement is usually met with a specially clean steel from which nonmetallic inclusions are eliminated to the limit. The higher is the steel strength, the higher is the possibility of steel experiencing fracture and fatigue due to nonmetallic inclusions. Consequently, there are more stringent requirements for reduction (in amount and size) of nonmetallic inclusions leading to fracture.
There have been proposed a variety of techniques to reduce the amount and size of hard nonmetallic inclusions in steel. For example, Non-patent Document 1 given below mentions that it is possible to improve fatigue characteristics by making steel to contain CaO—Al2O 3—SiO2 inclusions having a melting point of about 1400 to 1500° C. and that such inclusions do not start fatigue fracture.
Moreover, Patent Documents 1 and 2 given below disclose an ultra clean steel excelling in fatigue characteristics which is produced in such a way that nonmetallic inclusions are sufficiently elongated at the time of hot rolling.
Also, Patent Documents 3 and 4 given below disclose a Si-deoxidized steel in which inclusions are elongated and made smaller in size by means of alkali metal compounds.
In addition, Patent Document 5 given below discloses a technique to reduce the amount of inclusions as well as the sectional area of inclusions at the time of hot rolling by lowering the melting point.
Non-patent Document 1
    The 126th and 127th Nishiyama Memorial Technical Lecture, Japan Iron and Steel Association, pp. 145-165.Patent Document 1
Japanese Patent Publication No. Hei-6-74484
Patent Document 2
Japanese Patent Publication No. Hei-6-74485
Patent Document 3
Japanese Patent Laid-open No. 2002-167647
Patent Document 4
Japanese Patent No. 2654099
Patent Document 5
Japanese Patent Publication No. Hei-7-6037